Kitamura et al U.S. Pat. No. 4,525,841 discloses a buried hetero-structure semiconductor laser diode comprising a semiconductor substrate of a first conductivity type. On top of the substrate, a first cladding layer of a first conductivity type is formed followed by an active semiconductor layer and a second cladding semiconductor layer of a second conductivity type. The active semiconductor layer has a narrower bandgap than the first and second cladding layers. The buried hetero-structure has a stripe geometry active region, with channels formed along both sides of the stripe and extending down to the first cladding layer. Thus, the channels formed in the double hetero-structure define a mesa which includes a small portion of the first or lower cladding layer, a portion of the active layer and a portion of the second or upper cladding layer. Current blocking layers are formed in the channels and on the portion of the double hetero-structure outside the channels, but not on top of the mesa located between the channels. The blocking layers serve to confine the current flow to that portion of the active layer incorporated into the mesa.
Such a structure may be manufactured as follows. The double hetero-structure is formed by successively growing on a (100) oriented n-InP substrate an n-InP cladding layer (5 microns thick, Sn-doped, 1.times.10.sup.18 cm.sup.-3) a non-doped In.sub.0.72 Ga.sub.0.28 As.sub.0.61 P.sub.0.39 active layer (0.1 micron thick) and a p-InP cladding layer (1 micron, thick Zn doped, 1.times.10.sup.18 cm.sup.-3). The active layer corresponds to an emitting wavelength of 1.3 microns. The double hetero-structure wafer is treated with a usual photoresist and chemical etching process to form two parallel channels in the &lt;011&gt; direction with a mesa stripe therebetween. The channels extend into the n-InP cladding layer. The mesa stripe may be 1.5 microns wide in the active layer portion and the channels may be about 10 microns wide and 3 microns deep.
Epitaxial growth is carried out on the etched semiconductor wafer to successively form a p-type blocking layer and an n-type blocking layer on the surface of the semiconductor wafer except for the top surface of the mesa. A p-InP embedding layer and a quaternary electrode layer are subsequently formed throughout the entire wafer surface.
The p- and n- InP blocking layers are produced by liquid phase epitaxial growth employing a two-phase solution in which a single InP crystal floats in an In growth melt. Selective epitaxial growth (i.e., no growth on the mesa) is achieved because the growth proceeds so fast at the sides of the mesa that phosphorus a minor atom contained in the melt, has a relatively high concentration at the sides of the mesa and is relatively depleted near the top of the mesa. Such a growth process is disclosed in Mito et al U.S. Pat. No. 4,425,650. After the embedding growth, the wafer is processed into the desired buried hetero-structure laser diode by forming electrodes and a pair of reflective end surfaces which function as a resonator by the cleavage of (011) surfaces followed by pelletization.
As a result of the two phase liquid phase epitaxy growth process used to form the blocking layers, the ends of the blocking layers are pinned adjacent to the top of the mesa and, are vertically separated from the active layer portion of the mesa by the height of the upper p-type cladding layer. Such a structure has an undesirably high leakage current.
It is an object of the present invention to provide a buried hetero-structure laser diode in which the vertical distance between the ends of the current blocking layers adjacent to the top of the mesa and the active layer portion of the mesa is smaller than the thickness of the upper cladding layer so that substantially reduced leakage current results.